International study on the validation of models for the environmental transfer of tritium and carbon-14.

The International Atomic Energy Agency (IAEA)'s Environmental Modelling for Radiation Safety (EMRAS) model evaluation programme includes a Tritium and Carbon-14 Working Group (TCWG), the goal of which is to test the performance of models for the environmental transfer. This paper describes work on two of the nine TCWG scenarios; one involved the prediction of time-dependent tritium concentrations in freshwater mussels subject to an abrupt change in ambient tritium levels and the second was concerned with the prediction of carbon-14 concentrations in rice grown in the vicinity of sources of continuous atmospheric releases.

Carbon-14 transfer into rice plants from a continuous atmospheric source: observations and model predictions.

Carbon-14 (14C) is one of the most important radionuclides from the perspective of dose estimation due to the nuclear fuel cycle. Ten years of monitoring data on 14C in airborne emissions, in atmospheric CO2 and in rice grain collected around the Tokai reprocessing plant (TRP) showed an insignificant radiological effect of the TRP-derived 14C on the public, but suggested a minor contribution of the TRP-derived 14C to atmospheric 14C concentrations, and an influence on 14C concentrations in rice grain at harvest. This paper also summarizes a modelling exercise (the so-called rice scenario of the IAEA's EMRAS program) in which 14C concentrations in air and rice predicted with various models using information on 14C discharge rates, meteorological conditions and so on were compared with observed concentrations. The modelling results showed that simple Gaussian plume models with different assumptions predict monthly averaged 14C concentrations in air well, even for near-field receptors, and also that specific activity and dynamic models were equally good for the prediction of inter-annual changes in 14C concentrations in rice grain. The scenario, however, offered little opportunity for comparing the predictive capabilities of these two types of models because the scenario involved a near-chronic release to the atmosphere. A scenario based on an episodic release and short-term, time-dependent observations is needed to establish the overall confidence in the predictions of environmental 14C models.

The transport mechanisms of (222)Rn in soil at Tateishi as an anomaly spot in Japan.

The (222)Rn concentration profiles in soil have been measured at an anomaly spot in Tateishi, Japan. In winter, the concentrations were low and showed a negative gradient with depth, but in other seasons, the concentration had both positive and negative gradients with depth, and dramatically changed by time. On the assumption that there was ventilation in deep layers and with driving forces of wind and temperatures, these phenomena were successfully explained. This finding would contribute to a numerical model for (222)Rn transport in soil.

A new method for determining HT deposition velocity and dependence of the velocity on environmental factors.

A new method was developed to determine HT deposition velocity and to investigate how environmental factors affect the deposition processes including the diffusion into soil and the biochemical conversion by microorganisms to HTO in soil. This method, using a chamber and two tritium samplers, enables determination of the HT deposition velocity in situ without a large scale experiment and an additional tritium source. The main problem concerning the use of a flow-through method is the possible effect of flow rate on flux measurement. Since the tritium sampler used in this study has many collecting columns, the flow rate is limited. Experimental results showed no significant effect of flow rate over a low range from 0.48 to 0.95 l.min-1. From the increase of HT deposition velocity with increase of water content, it was indicated that biochemical oxidation rather than physical diffusion dominantly affects the HT deposition velocity under examined site conditions. No clear relationship between the HT deposition velocity and atmospheric temperature, relative humidity or soil temperature was observed. The soil temperature was only of secondary importance as a factor controlling the HT deposition velocity.